The present invention relates to a cradle for supporting a subject (human subject) in an X-ray CT system that obtains an X-ray tomographic image of the subject by means of X-ray exposure.
An X-ray CT (computerized tomography) system and apparatus comprises an apparatus (generally referred to as a gantry apparatus) having a toroidal cavity portion therein, and an operating console for supplying several types of control signals to the gantry apparatus and reconstructing an X-ray tomographic image for display based on signals (data) acquired by the gantry apparatus.
The gantry apparatus has an X-ray source (X-ray tube), an X-ray detector for detecting X-rays, and a cavity for disposing a subject therebetween. By rotating the X-ray source and X-ray detector, signals (data) are obtained at different rotation angles corresponding to the amount of X-rays which has passed through (and been attenuated by) the subject. In response to the signals, the operating console arithmetically calculates the X-ray attenuation factor in a small portion in a cross-sectional plane through the subject, and displays the calculated value as a pixel value to ultimately create an image visible by a human observer. The image is generally referred to as an X-ray tomographic image, and the process of creating the X-ray tomographic image is referred to as an X-ray tomographic image reconstruction process, or more simply, as reconstruction.
In addition to the above components, the X-ray CT system requires a carrier apparatus for supporting and immobilizing the subject in the cavity portion in the gantry apparatus and for carrying the subject toward the cavity portion. A table which is provided over the carrier apparatus and with which the subject comes in direct contact generally referred to as a cradle.
Since the cradle, along with the subject, is exposed to X-rays, the material constituting the cradle is required to have a high transmission factor to X-rays. In general, a material comprising a foam material, such as acrylic resin, reinforced by a surrounding CFRP (carbon fiber-reinforced plastic) or the like is employed.
In the X-ray CT system, noise images, referred to as artifacts, sometimes appear on a reconstructed image. An artifact occurs when an object has a higher X-ray attenuation factor (a lower X-ray transmission factor) than its surroundings. If the noise occurs outside the reconstructed X-ray tomographic image of a subject, it causes no problem. However, a noise occurring inside the reconstructed image may lead to misdiagnosis.
It is also known that the artifact phenomenon is pronounced in the tangential direction of an object having a high X-ray attenuation factor.
The relationship between a conventional cradle configuration and artifacts will now be described.
FIG. 1 shows three-direction projection views of a cradle around its end portion on a side near the gantry apparatus, in which (a) is a top plan view of the cradle (a view from upside of a subject, if placed on the cradle), (b) is a front (end) view from the carrying direction (i.e., from the gantry apparatus), and (c) is a side view.
As shown, the cross section of the cradle as viewed from the cross section or from the carrying direction has a bowl-like shape (inverted trapezoidal shape) whose top periphery forms a concave curve, as shown in the end view (b), so that the lying subject is stably supported. Moreover, the cross section as viewed from the lateral side is formed as a shape cut at an end at an angle xcex8, as shown in the side view (c). (The reason for the cut shape will be described later.)
FIG. 2 shows an exemplary X-ray tomographic image reconstructed by laying a subject on the illustrated cradle and scanning the subject at a position S shown in FIG. 1. (The scan plane is orthogonal to the drawing plane of FIG. 1.)
Although the cradle is made from a material with a high X-ray transmission factor, the reinforcing CFRP constituting the surface of the cradle has a lower transmission factor than the ambient air. Accordingly, many linear artifacts are generated along the tangential direction of the CFRP covering the surface, as shown in FIG. 2.
Since the cross section of the cradle has a bowl-like shape, the artifacts generated at the lateral surfaces of the cradle appear only in a direction away from the subject. Similarly, the artifacts generated at the curved surface on which the subject is placed appear only in the tangential direction and do not enter the X-ray tomographic image of the subject. In these points, it may safely be said that these artifacts substantially do not affect a diagnosis based on the X-ray tomographic image in such a condition as shown in FIG. 2.
Next, the reason for the shape of the end portion cut at an angle of xcex8 as viewed from the lateral side shown in FIG. 1 will be explained, and then problems about artifacts at the end portion will be discussed.
A principal cause of artifacts is the existence of a material with a low X-ray transmission factor in a scan plane. Therefore, if the end surface of the cradle is not slanted but, unlike that shown in FIG. 1, is perpendicular, artifacts are generated when a position that coincides with the perpendicular end surface is scanned, because a CFRP layer having a low X-ray transmission factor extends in a plane that coincides with the scan plane. Hence, the surface at the end portion of the cradle is made slanted as shown in the side view FIG. 1(c).
The angle xcex8 should be larger than a certain value. This is because a scan may possibly be performed with the gantry apparatus tilted by a certain angle xcex80, rather than always being performed in a plane orthogonal to the carrying direction of the cradle, as shown in FIG. 3. The tilt is made because, for example, a range of vertebrae constituting the backbone of a human subject extends in a gentle S-shaped curve and an X-ray tomographic image sometimes needs to be reconstructed in a plane orthogonal to a certain portion in such a curved range. Therefore, if the slope angle xcex8 at the end portion of the cradle is smaller than the maximum tilt angle, there occurs a situation in which the tilt angle of the gantry is equal to xcex8 in an actual use, causing those artifacts described above to be generated. Hence, the slope angle xcex8 of the end surface of the cradle is required to be larger than the maximum tilt angle of the gantry apparatus. However, if the gantry apparatus does not have a tilt function, a moderate angle will do without the above limitation on the angle.
Moreover, when a subject is laid on the cradle, the end portion may support either the head or the feet of the subject. Since a scan requires a stable condition of the subject, some cradles include a mechanism at the end portion for stably securing the head of the subject. This is achieved by inserting a head rest into an attachment slot provided at the end surface of the cradle, as shown in FIG. 4.
Consider a case of performing a scan at a position, designated by reference symbol S in FIG. 5, in the carrying direction (generally referred to as the Z-axis).
In the illustrated case, a scan plane intersects two portions Pa and Pb at the corners of the cradle. The cross section of the scan plane is shown at xe2x80x9cS-cross sectionxe2x80x9d in FIG. 5, and two end cross sections Pa and Pb of the cradle are formed.
As can also be seen from the enlarged view of the cross sections Pa and Pb of the end portion of the cradle, bottom peripheries Pa1 and Pb1 of these cross sections (which are in a layer of CFRP) are both horizontal, and in addition, are collinear. Consequently, artifacts arc generated, or tend to be generated, on a line connecting the bottom peripheries Pa1 and Pb1. Therefore, if a certain site of the subject is placed at the end portion of the cradle, the artifact connecting the bottom peripheries Pa1 and Pb1 enters the X-ray tomographic image, as shown in FIG. 5, preventing an accurate diagnosis.
The present invention was made in consideration of such problems, and is directed to providing a cradle and an X-ray CT system for reducing artifacts entering an X-ray tomographic image of a subject in performing a scan at an end portion of the cradle, and offering a reliable diagnosis environment.
In order to solve such problems, a cradle for an X-ray CT system of the present invention has, in one embodiment, the following configuration:
a cradle for placing thereon a subject in an X-ray CT system and for carrying the subject toward a scan position:
which has a top surface for placing the subject thereon that forms a concave curve as viewed from the carrying direction;
which has a generally bowl-like shape in a cross section as viewed from the carrying direction; and
representing points at the same position with respect to the carrying direction on two peripheries along which the top surface and two lateral surfaces abut as A and B, and n points on a minimal curve connecting Points A and B on the top surface as C1, . . . , Cn,
which has at an end portion a shape that is cut in a plane passing through Points A and C1 and descending toward a direction opposite to the carrying direction, is cut in a plane passing through Points Ci and Ci+1 and descending toward a direction opposite to the carrying direction, and is cut in a plane passing through Points B and Cn and descending toward a direction opposite to the carrying direction.
According to the present invention as described above, artifacts entering an X-ray tomographic image of a subject can be reduced in performing a scan at an end portion of a cradle, and a reliable diagnosis environment can be offered.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.